This invention relates to a pumping-up hydroelectric power plant utilizing a single speed reversible pump/turbine and a booster pump operable in series when the pump/turbine is operated in the pumping mode.
Ordinarily, the single speed reversible pump/turbine has performance characteristics wherein the maximum efficiency points for the turbine operation and the pumping operation are different from each other. More specifically, the total head corresponding to the maximum efficiency operating point in the pumping mode of the single speed pump/turbine is considerably lower than the turbine net head corresponding to the maximum efficiency operating point in the turbine mode of the single speed pump/turbine. However, in an actual hydroelectric power plant, because of the friction loss in the waterpassage between the upper reservoir and the lower reservoir, the total head against which the pump/turbine is operated as a pump is larger than the turbine net head under which the pump/turbine is operated as a turbine. For this reason, when the pump/turbine has been designed to be operable in the turbine mode at a maximum efficiency under a head condition of the hydroelectric power plant, the efficiency of the same pump/turbine is remarkably reduced when the pump/turbine is operated in pumping mode under the same head condition.
In order to overcome this difficulty of the single speed reversible pump/turbine, there has been proposed a two speed pump/turbine which rotates for the pumping operation at a speed higher than that of the turbine operation. Although the two speed pump/turbine is advantageous in that it is operable at maximum efficiencies in both the turbine mode and the pumping mode under the same head condition of the hydroelectric power plant, the pump/turbine is found to be disadvantageous because of difficulties in the design and construction of the generator-motor directly coupled thereto.
For obviating the above described disadvantages, there has been proposed an arrangement comprising a single speed pump/turbine and a booster pump provided in a bypass portion on the draft tube side of the same pump/turbine so that the pump/turbine and the booster pump are operated in series against a head condition of the power plant.
An example of such arrangement is shown in FIG. 1, in which numeral 1 designates the single speed pump/turbine directly coupled with a reversible generator-motor 2. A penstock 3 connects the reversible pump/turbine 1 with an upper reservoir (15), whereas a draft tube 4 and a draft tunnel 5 connect the reversible pump/turbine 1 with a lower reservoir 6. A water regulator 13 such as a guide vane is ordinarily provided between the penstock 3 and the pump/turbine 1.
A by-pass conduit 7 is provided for by-passing one portion of the draft tunnel 5, and a booster pump 9 directly coupled with a driving motor 8 is provided in the by-pass conduit 7 extending between two branching points. The by-pass conduit 7 may otherwise be so constructed that one end of the conduit 7 remote from the main pump/turbine 1 is further extended to open in the lower reservoir 6.
In the above described arrangement, a transfer valve 10 is further provided at a branching point, near the pump/turbine 1, of the by-pass conduit 7, while a stop valve 11 is provided in the portion of the draft tunnel 5 by-passed by the by-pass conduit 7. Furthermore, guide vanes (not shown) are provided in the penstock 3 at an end thereof adjacent to the pump/turbine 1.
When the reversible pump/turbine 1 is operated in the turbine mode, the transfer valve 10 and the stop valve 11 are operated to close the by-pass conduit 7 and to open the draft tunnel 5 respectively, and the guide vanes are opened to cause the water in the penstock 3 to flow into the pump/turbine 1.
The pump/turbine 1 is then driven in the turbine mode to operate the generator-motor 2 directly coupled therewith in the generator mode. The water discharged from the pump/turbine then flows through the draft tube 4 and the draft tunnel 5 into the lower reservoir 6.
In a case where the reversible pump/turbine 1 is operated as a pump, the transfer valve 10 and the stop valve 11 are operated to open the by-pass conduit 7 and to close the by-passed portion of the draft tunnel 5. Then, the generator-motor 2 directly coupled to the pump/turbine 1 and the motor 8 for driving the booster pump 9 are both energized to operate the pump/turbine 1 and the booster pump 9 in series so that the water in the lower reservoir 6 is pumped-up through the by-pass conduit 7, draft tunnel 5, draft tube 4, and the penstock 3 into the upper reservoir (not shown).
In FIG. 2, there are indicated performance characteristics of the main pump/turbine 1 in the pumping mode, the booster pump 9, and the series combination of the pump/turbine 1 and the booster pump 9, with total head H and efficiency .eta. indicated along the ordinate and water discharge Q indicated along the abscissa.
More specifically, a curve L.sub.a represents a variation in the total head H of the main pump/turbine when the latter is operated in pumping mode at the rated speed; a curve L.sub.b represents a variation in efficiency .eta. of the main pump/turbine 1 when the same is operated under the same condition; and a curve L.sub.c represents a variation in the total head H of the booster pump 9 when the latter is operated at a rated speed N.sub.b0. Furthermore, a curve L.sub.d represents a variation in the total head H of the series combination of the turbine 1 and the booster pump 9, where the booster pump 9 is operated at the rated speed N.sub.b0.
Assuming that H.sub.1 represents a total head required for pumping-up water from the lower reservoir to the upper reservoir, a horizontal line H=H.sub.1 intersects the curve L.sub.d representing the series combination of the main pump/turbine 1 and the booster pump 9 at a point P.sub.1 corresponding to the operating point of the hydroelectric power plant. Since the two machines 1 and 9 are operated in series, the quantity of water discharged from these machines is equal to Q.sub.1 along the abscissa corresponding to the point P.sub.1. Thus, the operating points of the main pump/turbine 1 and the booster pump 9 are represented by P.sub.m1 and P.sub.b1, respectively, while the efficiency of the main pump/turbine 1 operating at the point P.sub.m1 is made to be approximately equal to the maximum value .eta..sub.m1. Representing the total heads corresponding to the operating points P.sub.m1 and P.sub.b1 by H.sub.m1 and H.sub.b1 respectively, the sum of the total heads H.sub.m1 and H.sub.b1 is of course equal to H.sub.1.
Comparing the efficiency .eta..sub.m1 of this case with an efficiency .eta..sub.m0 corresponding to the operating point P.sub.m0 where only the pump/turbine 1 is operated against the total head H.sub.1, it is apparent that the efficiency in the pumping operation of the pump/turbine 1 is remarkably improved by operating the pump/turbine 1 in series with the booster pump 9.
This arrangement, however, has a difficulty in that when the total head of the power plant varies from the above described value H.sub.1 to a value H.sub.2, the operating point of the power plant is shifted from the above described point P.sub.1 to another intersecting point P.sub.2 between the curve L.sub.d and a horizontal line H=H.sub.2.
As a result, the operating points of the main pump/turbine and the booster pump are shifted to P.sub.m2 and P.sub.b2 respectively, and the efficiency of the main pump/turbine is varied from .eta..sub.m1 to .eta..sub.m2 which is considerably lower than .eta..sub.m1.
In other words, the arrangement of the hydroelectric power plant comprising a main pump/turbine and a booster pump operable in series for sharing the total head of the hydroelectric power plant therebetween has a difficulty in that the efficiency of the main pump/turbine is reduced when the total head of the power plant is varied by, for instance, a variation in the static head between the upper reservoir and the lower reservoir.